1. Field of the Invention
This invention relates to the restoration of catalytic activity of hydrodesulfurization catalysts that have become deactivated in service due to the deposition thereon of vanadium and nickel deactivants.
2. Description of the Prior Art
Many hydrocarbon feedstocks, such as 650.degree. F + atmospheric residual oils or 900.degree. F + vacuum residual oils, must be refined by hydroprocessing methods to obtain a product oil of improved characteristics. One such method is hydrodesulfurization, a process primarily to reduce the sulfur content of the oil but which also may reduce the nitrogen content, depending on the severity of the operating conditions. Hydrodesulfurization is accomplished by contacting the residual feedstock at elevated temperatures (about 550.degree.-780.degree. F) and pressures (about 600-2500 psig) with catalyst particles comprising one or more Group VIB and/or Group VIII active components on a refractory oxide support. The contacting is done in the presence of hydrogen and (usually in the presence of steam) under conditions selected to produce a product oil substantially reduced in nitrogen and sulfur content.
A problem is encountered in hydrodesulfurizing most residual oils because such hydrocarbon feeds usually contain high concentrations of vanadium and nickel components (usually 50-1000 wppm of Ni + V calculated as the metals). During hydrodesulfurization, some of these vanadium and nickel components deposit upon the hydrodesulfurization catalyst, most probably in the form of organo-metallic compounds and/or one or more metallic sulfides. Deposited in such forms, nickel and vanadium deactivate the hydrodesulfurization catalyst in a manner that appears to involve plugging of the catalyst pores. It addition, these catalysts are further deactivated during hydrodesulfurization when coking of the catalyst occurs concomitantly with the deposition of nickel and vanadium deactivants. In extreme cases the catalyst loses essentially all its original activity because of coking and metals deposition.
The deactivation of hydrodesulfurization catalysts as described hereinabove is clearly undesirable. When a catalyst has lost a substantial proportion of its activity, hydrodesulfurization becomes more difficult. Increased operating temperatures and pressures become necessary to remove the required amount of sulfur and nitrogen from the hydrocarbon feedstock. This naturally increases the cost of producing valuable conversion products containing these elements in low concentrations. Thus, it is very desirable either to prevent the coke or metal deactivants from depositing on the catalysts or to devise a feasible method of restoring the activity of the catalysts after such deposition.
Much work has been done to discover an aqueous or organic solution which will restore the activity of hydrodesulfurization catalysts by removing vanadium and nickel components. In U.S. Pat. No. 3,791,989 issued to Mitchell et al. a process is disclosed for recovering catalytic activity of fouled hydroprocessing catalysts by (a) using a concentrated solution of oxalic acid to remove vanadium sulfide deactivants and then (b) de-coking the catalyst by an oxidative burn-off. No data, however, are given in this reference for the activity of treated catalysts following the practice of this method. In attempting to utilize this method on catalysts heavily deactivated during desulfurization, it has been found that oxalic acid dissolves alumina supports in environments severe enough to remove substantial amounts of vanadium. In some cases, little vanadium is removed even when high concentrations of oxalic acid are used at elevated temperatures.